Leonard is losing at Sheldon's hand in a game of 3-Dimensional Chess . Penny has just returned from Nebraska and stops by for her mail, she adds "It was the worst trip. Everyone got sick over the weekend." Hypochondriac Sheldon springs up and distances himself from Penny and starts spraying the area, as he is afraid he may catch a cold. Leonard quickly dismisses this action as another one of Sheldon's overreactions.
Later that night, Sheldon asks Leonard to swab his throat to see what disease is infecting him. Leonard refuses and says he's going back to bed, then Sheldon asks him to place a measuring cup in the bathroom so Sheldon can measure his urine output to make sure his kidneys are not shutting down. Leonard quickly becomes angry because he mixes pancake batter in that cup. After Sheldon proves to Leonard that the measuring cup was always used to measure urine, Leonard sarcastically says he owes the Betty Crocker company a letter of apology. By the next morning, Sheldon discovers he is sick.
Leonard, realizing that Sheldon was telling the truth, quickly mobilizes and dashes out of the apartment to avoid Sheldon and having to wait on him hand and foot. When Sheldon calls to find him, Leonard lies, telling Sheldon that he got called in to work at the most unusual time (at 6:30 in the morning on a Sunday). When Sheldon hears a dog in the lobby barking in the background, Leonard makes up an excuse about dogs being trained in the lab to operate centrafuges before hanging up and calling Howard. Leonard tells Howard to call Raj to let him know that they need to avoid Sheldon for a day. Howard tries to convince his mom to invite Leonard and Raj, but she refuses, claiming she just had the carped steamed. Howard says there is a "Planet of the Apes" movie marathon (showing five movies, each lasting two hours), so Leonard, Howard and Raj decide to go to there to lay low and avoid Sheldon.
With no one to wait on him, Sheldon goes to the Cheesecake Factory for soup. Even with his 187 IQ, Penny has to point out that Sheldon could have gotten soup delivered, rather than venture out. Sheldon asks about split pea soup with hot dog slices and croutons like his mother used to make. Penny advises that they have potato-lentil soup or chicken tortilla soup. Sheldon says: "surprise me". As Penny goes to get his soup, he blows his nose into his napkin and turns to a family sitting at a table next to him, asking, "...would you call this moss green or forest green?"
At the screening, Leonard, Howard, and Raj, are wearing chimpanzee masks, only to find everyone in the cinema are also wearing the same masks as them, much to Howard's annoyance. Raj states that he wanted to wear an orangutan mask but they shouted him down. Just as they are about to take their seats, Leonard's phone rings. He answers and finds Penny calling him. He again lies, telling Penny he's at work, wearing a radiation suit (to explain the mask distorting his voice). When Penny mentions Sheldon is at the Cheesecake Factory, Leonard said to Penny to tell Sheldon to head back to the apartment. Penny replies, saying that Sheldon is afraid he'll pass out on the bus and someone will harvest his organs and she pleads to Leonard to come get him. Leonard accepts, only to pretend that a nuclear meltdown is imminent (with Howard and Raj imitating sirens) so he can hang up. He then feels guilty for it, Raj tries to comfort him while trying to steal Howard's popcorn, only for Howard to decline him, yelling "Take your stinking paws off my popcorn you damn, dirty ape!"
After Penny drops Sheldon off at his apartment after leaving work early, out of sympathy she gets pulled into taking care of him. He tells her that when he was a fifteen-year-old visiting professor in Germany, he got sick. His mom flew back to Texas to help his dad with their house, as it was aluminum [i.e., a pre-fabricated or mobile home] and had slid off the cinder blocks again. He told her that a maid gave him an enema.
Back at the screening, just as the ten and a half hour marathon starts, Howard accidentally breaks Leonard's glasses after Leonard had dropped them, rendering him all but sightless. At Sheldon's apartment, Sheldon begs Penny to rub some Vicks VapoRub on his chest; he also asks her to sing Soft Kitty to him, a song his mother always sang to him when he was sick.
As Leonard doesn't want to watch the marathon blurred by his virtual blindness due to breaking his glasses, the three go back to the apartment in an attempt to retrieve Leonard's spare glasses, which are in his bedroom. Howard and Raj use a Bluetooth camera mounted on Leonard's head to help blind Leonard navigate, as well as a detector to let them know if/when Penny appears. Leonard is caught by Penny, at which point she calls him a "Rat Bastard". Howard and Raj quickly abandon him, running down the stairs. An angered Penny alerts Sheldon of Leonard's presence. Leonard, still blinded, makes a mad dash to escape, but runs into one of the apartment's pillars. Finally, on the couch, Leonard sits with an ice bag on his head, next to a still-sick Sheldon. Sheldon asks Leonard to help him but Leonard implies he stood up at least twice, saying he can't stand without puking.
- Guest starring:
- Teleplay: Bill Prady & Stephen Engel
- Story: Chuck Lorre & Lee Aronsohn
- "A very dull episode. The humor is built around Sheldon being annoying and it just isn't convincing or funny." - The TV Critic's Review
- IMDb user reviews
- Title Reference: The title refers to the anomalous cross-contamination between Sheldon's urine cup and Leonard's pancake batter.
- Chuck Lorre's vanity card 
- This episode was watched by 8.68 million people with a rating of 3.1 (adults 18-49).
- Episode transcript .
- Sheldon revealed that Leonard is a poor chess player in the previous episode (S1E10) and it is shown how he is not well suited for three-dimensional chess in this episode. Penny can also beat Leonard at chess in "The Werewolf Transformation" (S5E18). Leonard would do well to read the Federation Standard Tri-D Chess Rules.
- The pseudoscalar meson nonet  is clearly seen on the small refrigerator blackboard, which is what Eric Gablehauser was "doodling" in "The Luminous Fish Effect" (S1E4).
- Code Milky Green is the emergency code for when Sheldon is sick, possibly cause of the color of mucus build up.
- "Möchtest Du eine Darmspülung?" Translates into "Do you want an enema?". Though it should be noted that while technically not wrong, the medically correct German term would be "Einlauf", not "Darmspülung".
- When Sheldon comes out looking for Leonard, he drops the duvet he is dragging to the floor before he enters the kitchen. In the next shot as he approaches the refrigerator, watch how the comforter has crawled down the step on its own.
- The song "Soft Kitty" is introduced. Penny learns it and sings it to sick Sheldon and many other times during the series.
- Sheldon states he has an IQ of 187, and in "The Codpiece Topology" (S2E2), he says he has an IQ which can't be accurately measured by normal tests and he'd have to lose 60 IQ points to be classified as smart. If Sheldon's IQ is indeed 187, then Leonard's IQ would be 173, as Leonard said their combined IQ is 360 in "Pilot" (S1E1).
- Sheldon claims that this time is only the second time he has been sick by himself. The first was when he was fifteen, at the Heidelberg Institute in Germany as a visiting professor (This very institute does not exist. What is meant here is the renowned "Heidelberg University" with its many diverse institutions). The sausage based cuisine causes "an intestinal blitzkrieg", and his mother had to return to Texas to help deal with their aluminum house slipping off its cinder blocks during tornado season. In the series "Young Sheldon" which takes place before Sheldon went to Germany, they do not live in a mobile home.
- Though Sheldon doesn't receive his first doctorate until he turns sixteen, he's a visiting professor at fifteen. This anomaly is not implausible, as exemplified by the case of real-life prodigy Alia Sabur. Also, there are famous physicists who obtained professorships or postdoctoral fellowships while holding non-doctoral degrees, such as Satyendra Nath Bose, Freeman Dyson and Lars Onsager.
- Leonard advises Sheldon to "get some rest and drink plenty of fluids". With characteristic appreciation and charm, Sheldon responds, "What else would I drink? Gases? Solids? Ionized plasma?" These are the four states of matter.
- Even under the stress of illness, Sheldon retained his rapier wit sufficiently to crack a "physics funny". It's "common knowledge" from Physics 101 class that gases and plasma are also fluids, just like liquids, and make refreshing "virgin" drinks.
- Technically, the Continuum mechanics#Concept of a continuum|applicability of a fluid approach for a collection of free particles (gases) lies in comparing the Mean_free_path_collisional|mean free path () to the macroscopic length scale () of interest. For ionized gases, the subject goes by the name magnetohydrodynamics when the methods of continuum mechanics apply, and by plasma when they do not.
- The domed building visible from Leonard and Sheldon's apartment 4A window is the Pasadena City Hall.
- When Sheldon was trying to test if he was sick, he had to use lime Jell-O|Jello to make petri dishes because "SOMEONE polished off the apricot yogurt," implying it was Leonard. While Leonard is lactose intolerant, if his intolerance is mild or moderate, he could eat yogurt if it had live active cultures.
- This is the first time that Penny calls Leonard a "Rat Bastard." The second time she does it is in the Season 3 finale of "The Lunar Excitation" (S3E23).
- In this episode, Sheldon said at the age of fifteen, he got sick in Germany, and his mother had to fly back to Texas to help his dad at that time. This creates a continuity error in the episodes "The Thanksgiving Decoupling" (S7E9) and "The Proton Transmogrification" (S7E22) because Sheldon mentions to Bernadette's father Mike and the ghostly form of Professor Proton, respectively, that his father died when he was only fourteen.
- On the TV version of the episode, the cold open ends which Penny closing the door and Sheldon using a disinfectant spray, but on the Season 1 DVD, an extended scene shows Leonard looking at him and Sheldon says "What?!"
- Before the show began, presumably, Sheldon had an ear infection in 2006, which is mentioned by Howard when on the phone to Leonard, as "the Great Ear Infection of '06".
- (Howard is asleep in his bed, when the phone rings.)
- Mrs Wolowitz: (1st time: she's shouting from downstairs off-screen) HOWARD, IT'S THE PHONE!
- Howard: (he yells in giant fury) I KNOW IT'S THE PHONE, MA! I HEAR THE PHONE!
- Mrs Wolowitz: (2nd time: she's shouting from downstairs off-screen) WELL, WHO'S CALLING AT THIS UNGODLY HOUR?!
- Howard: I DON'T KNOW!
- Mrs Wolowitz: (3rd time: she's shouting from downstairs off-screen) WELL, ASK THEM WHY THEY'RE CALLING AT THIS UNGODLY HOUR!
- Howard: HOW CAN I ASK THEM WHEN I'M TALKING TO YOU?!
- Howard: [Answers the phone quietly] Hello?
- Leonard: (in the apartment lobby, hiding from Sheldon, and fastening his shoelaces.) Hey Howard, it's Leonard. Code: Milky Green.
- Howard: Dear Lord, not Milky Green.
- Leonard: Affirmative, with fever.
- Mrs. Wolowitz: (4th time: she's shouting from downstairs off-screen) WHO'S ON THE PHONE?!
- Howard: IT'S LEONARD!
- Mrs. Wolowitz: (5th time: she's shouting from downstairs off-screen) WHY IS HE CALLING?!
- Howard: SHELDON'S SICK!
- Mrs. Wolowitz: (6th time: she's shouting from downstairs off-screen) WERE YOU PLAYING WITH HIM?!
- Howard: FOR GOD'S SAKE, MA, I'M 26 YEARS OLD!
- Mrs. Wolowitz: (7th time: she's shouting from downstairs off-screen) EXCUSE ME, MR. GROWN-UP! WHAT DO YOU WANT FOR BREAKFAST?!
- Howard: CHOCOLATE MILK AND EGGOS, PLEASE!
- Leonard: Howard, listen....
- [Phone beeps.]
- Howard: Hang on, call waiting.
- Leonard: Wait, NO, don't -
- [Howard takes the call, unaware that it's Sheldon.]
- Howard: Hello?
- Sheldon: Howard, I'm sick!
- (Howard doesn't know what to do until he decides to pretend to be his mother answering.)
- Howard (as Mrs. Wolowitz, and sounding vaguely Australian): Uh, HOWARD'S SLEEPING, THIS IS HIS MOTHER! WHY ARE YOU CALLING AT THIS UNGODLY HOUR?!
- Sheldon: I need soup!
- Howard (as Mrs. Wolowitz): THEN CALL YOUR OWN MOTHER! (To Leonard; normal voice) It was Sheldon.
- Leonard: I tried to stop you.
- Howard: It’s my own fault, I forgot the protocol we put in place after the great ear infection of ’06.
- Leonard: You call Koothrappali, we need to find a place to lay low for the next eighteen to twenty four hours.
- Howard: Standby. (Goes over to his bedroom door and shouts) Ma, can my friends come over?
- Mrs. Wolowitz: (8th time: she's shouting from downstairs off-screen) I JUST HAD THE CARPETS STEAMED!
- Howard: ... (Back on the phone) That's a negatory. But there's a Planet of The Apes marathon at the Newark today.
- Leonard: Five movies, two hours apiece. It’s a start.
- [Raj steals some of Howard's popcorn.]
- Howard (doing an impression of Charlton Heston): Take your stinking paws off my popcorn, you damn dirty ape.
- Sheldon: Can you sing “Soft Kitty”.
- Penny: What?
- Sheldon: My mom used to sing it to me when I was sick.
- Penny: I’m sorry, honey, I don’t know it.
- Sheldon: I’ll teach you. “Soft kitty, warm kitty, little ball of fur, happy kitty, sleepy kitty, purr purr purr.” Now you.
- Penny: (after a loud sigh) Soft kitty, warm kitty…
- Sheldon: Little ball of fur. Keep rubbing.
- Penny: (through gritted teeth) Little ball of fur.